无法制作JSON多级PHP

I have a problem in creating a JSON file as I need it in my swift app

here is how my DB look like

    ID  | PICName   | PicPath       |DeviceName
 ---------------------------------------------------
    1   | NewPIC    | new.jpg       |ibrahiem
    2   | NewPIC    | new.jpg       |ibrahiem
    3   | NewPIC    | new.jpg       |ibrahiem
    4   | NewPIC    | new.jpg       |Ali
    5   | NewPIC    | new.jpg       |Ali
    6   | NewPIC    | new.jpg       |Ali

I need it to by JSON with 3 levels

DeviceName ---> ID ---- > ( PICName , PicPath)

I just now how to make simple JSON so please correct me if I have a mistake in some commas or brackets

this is my simple code

<?php

$con=mysqli_connect("127.0.0.1","erterte","534234","u13");
if (mysqli_connect_errno()) {
   echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

$return_arr = array();
$return_arr['xx']= array();
$query = "select * from Products order by deviceid";
$result = mysqli_query($con,$query);
$rows = array();

while($r = mysqli_fetch_array($result)) {
    $rows['ID'] = $r['ID'];
    $rows['PicPath'] = $r['PicPath'];
    $rows['DeviceName'] = $r['DeviceName'];
    $rows['PICName'] = $r['PICName'];

    array_push($return_arr['xx'],$rows);

}

echo json_encode($return_arr);
mysqli_close($con);
?>